The light-dependent
reactions of photosynthesis

Pass the mouse pointer over this diagram for more information.

The diagram above - based on the Z-scheme - suggests the amount of energy in each compound and also partly explains their location in the membranes as energy, electrons, and H+ are passed between them, rather like a pinball game.

This webpage is an updated version of this material, with some extra detail.
The previous version which refers to hydroxide ions as the source of electrons, as stated by some exam boards, is still available.

The light-dependent reactions take place in the membranous sections of the chloroplast - thylakoids (granae/lamellae) - which offer a large surface area to absorb light energy.

The main function of these reactions is to provide a source of ATP and reduced NADP, which are used to reduce CO2 in the light independent reactions.

There are two photosystems consisting of chlorophyll and associated molecules.

The first in the chain - photosystem II - absorbs light best at wavelength 680 nm and is called P680 (also sometimes called P690).

The second - photosystem I - absorbs light best at wavelength 700 nm and so is called P700. [Numbers II and I refer to the order of discovery, not position in chain.]

A number of chlorophyll molecules form an "antenna complex" which absorb light energy from photons, and pass on this energy to a reaction centre.

One P680 chlorophyll molecule at the reaction centre of photosystem II is raised to a high enough energy level to lose electrons (which makes it positively charged, oxidised).

This electron loss is repaid from electrons taken from water during the process of photolysis.

Photolysis is the splitting of water by light – the basis for non-cyclic photophosphorylation.

This requires a continuous supply of water as a raw material for the process.

The oxygen evolving complex (part of photosystem II) is effectively an enzyme that carries out the oxidation of water. It contains 4 manganese ions and 1 calcium ion as cofactors, which cycle through 4 oxidation-reduction states.

The photolysis reaction can be written as follows:

2H2O 4 H+ + 4e- + O2

Oxygen is released as a by-product.

Hydrogen ions (H+, protons) are released into the lumen of the thylakoid.

Here H+ ions build up (chemiosmotic effect, similar to mitochondria) and when H+ passes through ATP synthase into the stroma of the chloroplast, ADP + Pi are converted into ATP - (photo)phosphorylation.

This ATP is used in the light-independent stages of photosynthesis.

Electrons leaving chlorophyll in PSII have been excited (raised to a higher energy level).

They pass some of this energy on to a chain of electron carriers.
Some of the electrons' energy passed on is used to pump hydrogen ions across the membrane so that it accumulates in the lumen of the thylakoid. This further increases the H+ concentration there, eventually indirectly contributing to the production of ATP (see above). The electrons become less energetic, and are eventually passed to a chlorophyll in the next photosystem (PSI).

After P700 receives another photon, the electrons are given a second excitation - raising them to a higher energy level than before - and they are passed to another set of electron acceptors/carriers.

At the end of this chain, the electrons are passed to NADP, which acts as both an electron acceptor and a hydrogen ion acceptor. This results in the production of reduced NADP, which is used in the light-independent stages of photosynthesis.